You need to look at other things apart from DAU, too. How many writes, how complicated is the database (number of tables, average number of joins to answer a qu

You need to look at other things apart from DAU, too. How many writes, how complicated is the database (number of tables, average number of joins to answer a query), what are the availability requirements? And how far can you go with having the db on the same machine: how much can you scale vertically, what's the fastest storage you can get?